202 



University of California. 



[Vol. i. 



lute identity of the holocrystalline augitic facies of the two masses 

 is fair evidence that both are genetically connected, and were prob- 

 ably also at one time continuous across the space now occupied by 

 the serpentine dyke. 



Of the various smaller isolated masses of eruptive rock shown 

 on the map, all of which are regarded as apophyses from the same 

 igneous mass that gave rise to the sill, by far the most interesting is 

 that of Point Blunt. The waves have here washed away much of 

 the soil, and reveal the most unmistakable case of an intrusive con- 

 tact between the eruptive rock and the sandstone and chert. Por- 

 tions of the intruding rock, moreover, show in moderate perfection 

 the peculiar spheroidal structure described by the writer in the 

 basalt of Point Bonita, proving conclusively that such structure 

 cannot be rigidly restricted to surface flows, although it is still 

 believed that lavas exhibiting it must have been erupted under very 

 nearly surface conditions. The racliolarian chert has been broken 

 through, as well as the sandstone, and a prominent stack just south- 

 west of the point, is a mass of the spheroidal eruptive thickly 

 crowded with inclusions of the red and the green chert, which, how- 

 ever, exhibit but slight evidences of alteration. The mechanical 

 results of the intrusion upon the sandstone are glaringly apparent 

 in an intense local crushing, followed by much secondary calcifica- 

 tion, as well as in a darkening in color. The microscope shows 

 that important chemical alteration has also been brought about, the 

 description of which will follow in its proper place. 



The various smaller intrusions of the fourchite indicated upon 

 the map do not call for detailed description. An interesting fact 

 was observed, however, in the case of the two minute patches on 

 the shore just north of Quarry Point; both of these contain included 

 fragments of bright red chert, although the latter rock is not 

 exposed in situ except at some little distance to the west, and con- 

 siderably higher up on the hill. 



Pctrograpliic Character. — Throughout the greater portion of the 

 sill, as observed on the horseshoe-shaped exposure passing around 

 the island, the fourchite preserves a fairly uniform character. It is 

 rust-brown on weathered surfaces, but dark gray on fresh fracture 

 showing a compact texture, which under the lens appears to be fine 



